An example of a radial imaging system comprises a reflecting cone which redirects light received (basically) radially inwardly to a (basically) axial direction, for collection by an axially aligned camera.
One use of such an arrangement is part of a catheter camera, in which the cross section of a passageway in which the catheter is located is to be inspected. An example of the use of such a catheter camera is for analysis of the upper airway, for determining the causes of obstructive sleep apnea.
Obstructive sleep apnea (OSA) is the most common kind of sleep apnea, affecting up to one in eighteen people, and is characterized by the occurrence of pauses in breathing, or instances of shallow or infrequent breathing, during sleep. It is caused by blockage or obstruction of the oral cavity or upper airway, often due to loss of muscular tone induced by the onset of old age, or (temporary) by abuse of drugs or alcohol.
A range of therapies exist for treatment of OSA, the most common of which is positive airway pressure (PAP), in which a ventilator is used to deliver a stream of air through the airway at a controlled pressure, in order to hold open the airway. PAP is needed in more severe cases, where patients exhibit an apnea hypopnea index (AHI)>30. OSA patients may also suffer from daytime sleepiness and require therapy to prevent the development of comorbidities over the longer term. Mild-moderate OSA patients often have more difficulty adhering to PAP therapy because the disease burden is not as strong as in severe patients, and are therefore reluctant to submit to so invasive a therapy. In these cases, various alternative treatments exist, such as positional therapy, mandibular advancement (oral appliances), upper airway surgery and implantable devices.
In each of these therapies, however, it is important to understand which part(s) of the upper airway in particular is (are) causing obstruction, such that the therapy can be directed most effectively. This explains the interest in dynamic examinations of the upper airway preferably in a non-invasive way. One approach is to perform an examination of the airway non-invasively using acoustic reflectometry techniques. In such techniques, acoustic waves are propagated along the airway of the patient, by an emitter, via the mouth or nose, and reflections are listened for using a microphone adjacent to the emitter. It is possible, through algorithmic analysis of the detected reflections (see for example: Hoffstein, V., and J. J. Fredberg. “The acoustic reflection technique for non-invasive assessment of upper airway area.” European Respiratory Journal 4.5 (1991): 602-611.), to determine an estimate of the cross-sectional area of the examined airway as a function of distance from the emitter. From this, narrowing of the airway at particular locations can be identified, and the specific positions therefore of airway obstructions ascertained.
Reflectometry techniques however suffer the disadvantage that the accuracy of cross-sectional area estimations declines with distance from the emitter. This is compounded by acoustic leakage and also patient movements during the measurement process, which both act to further compromise the accuracy of the obtained results. Furthermore, since the first obstruction encountered by a wave propagating along the airway causes reflection of much of the wave's initial intensity, reflections from subsequent portions of the airway are typically too weak in intensity to derive any accurate measurements. Hence it is typically only possible to accurately determine the location of the upper-most airway obstruction using these techniques.
It is known instead to use endoscopic procedures, in particular procedures for inspecting or investigating the patency of the human upper airway. Using a standard flexible endoscope for airway examination, specific sites in the upper airway can be inspected for some time to see whether temporary obstructions occur. This however requires the endoscope to be moved from one spot to the other during an examination which is time-consuming and inconvenient for the patient. For this reason endoscopic examination during natural sleep did not become part of common practice. An alternative version which has acquired some acceptation in current practice involves bringing the patient to artificial sleep by means of sedative drugs. This is believed to cause collapses at sites that also participate in real sleep apneas and hypopneas. Also the sedation relieves the discomfort of endoscope travel.
To inspect the upper airway at some discrete critical sites, it is also possible to use a catheter with multiple image sensors; once the catheter has been inserted it can remain in the same position during a longer period without additional discomfort for the patient. The interpretation of the images acquired at multiple sites over a long period is very time consuming.
Image sensors can also be used to obtain a measure of radial distance, for example if a ring is illuminated around the inside of the airway, the captured image sensor information in respect of the ring image can be analyzed to derive distance information, and thereby enable the shape of the internal airway passage to be derived.
For example, an endoscope may have a light generating means capable of producing an outwardly directed ring (or radial plane) of light, such that when inserted into a tube-like airway, cross sectional contours of the airway may be illuminated for inspection by a camera.
One known means of providing such a light pattern is to direct collimated laser light from an optical fiber toward a deflecting cone whose angle is such as to deflect the incident light radially, for example at 90 degrees, from its surface in all directions around it. The effect is to create a ‘ring’ pattern of light projecting outwards from the cone, which may then be used to illuminate a circumferential section of an airway. In particular, there are two variations of this concept. In a first, the cone has a reflective outer surface, and is arranged with its tip facing in the direction of the oncoming light, such that light is reflected directly out from its surface. In a second, the cone is arranged with its base facing toward the oncoming light and the pitch arranged such that light incident from the optical fiber on the internal walls of the cone is reflected by total internal reflection in the direction of the opposing wall, through which it is transmitted, deflecting due to refraction as it does so into a path which is at 90 degrees to the initial incident light.
The reflected light is then captured by a camera. This may be achieved by positioning the camera with the inner wall being examined within the field of view, or else another reflecting cone may be used to redirect the reflected light back to an almost axial direction for capture by an axially aligned camera.
It is possible to create multiple ring patterns of light, at a series of spaced points along the airway. This can for example be achieved by means of providing multiple illumination units along the catheter, each with its own laser, optical fiber (optionally a GRIN lens) and cone.
This invention relates in particular to the reflector used to redirect the received incident radial light towards a camera (or any type of image sensor). A standard reflecting cone may be used, with a circular base and a tip (apex) which lies on the normal line through the center of the circle. The lateral surface of the cone is formed by straight line segments joining the apex to the perimeter of the base. This circular cone reflector is fully characterized by the distance of the tip to the base and the angle (μ) the straight lines connecting the perimeter to the tip make with the base plane. The angle at the tip is given by π−2μ.
The tip angle and the distance to the camera are chosen such that the camera captures all projected rings with a radius in a very specific range.
This arrangement has a problem that the sensitivity to changes in the radius of the rings depends strongly on the ring radius itself: the farther away the ring being imaged, the less sensitive. The reflecting cone arrangement is therefore not able to be effective over a large range of possible distances from the central axis to the wall of the duct under examination and it prevents uniform measurement accuracy.
Desired therefore is a simple optical arrangement which addresses these problems.